The invention is related to the fields of broadband cable television systems and is most closely related to laser optical communication links for such systems.
In a cable television system, television programs are provided at a central head-end. The programs are distributed from the head-end through optical fiber tree networks to a multitude of local nodes in respective communities, and then further distributed from the local nodes through coaxial cable tree networks to customer interface units (CIUs) also called cable terminations. Currently, many of these systems are beginning to provide other communication services such as telephone service and/or computer networking services (e.g. internet connection) through the cable television system. Telephone and computer networking services require bi-directional communication in the cable television system. Forward data signals are transmitted similarly to television signals, as described above, and return data signals are transmitted through the same path in the reverse direction. That is, return signals are collected from the CIUs through the coaxial cable tree networks to the local nodes, back through the local nodes, and back through the optical fiber tree network to the head-end.
At the head-end, a multitude of electronic forward information signals for the television, telephone, and computer communications are used to modulate respective carrier signals of different frequencies. The modulated carrier signals are combined into an electronic forward signal that is used to modulate a forward laser beam to produce an optical forward signal carried by the forward laser beam. The modulated laser beam, carrying the optical forward signal, is transmitted through an optical fiber tree network to a multitude of the local nodes. At each local node an optical detector converts the optical forward signal back into an electronic forward signal. The reconverted electronic forward signal is transmitted from the local nodes through a coaxial conductor tree network to CIUs at homes and businesses of customers.
Telephone and computer equipment of the customer, are connected to the CIUs by the customers and the customer""s equipment produces return signals that are transmitted by the CIUs into the coaxial tree. The return signals are multi-carrier modulated signals similar to the forward signals. The return signals travel back through the coaxial tree network to the local nodes. In the local nodes, the return signals are separated from the forward signals by diplex filters. The separated return signals are used to modulate a return laser beam to produce an optical return signal carried by the return laser beam. The optical return signal is transmitted back through the optical fiber tree network to the head-end where the optical return signals are converted back into electronic return signals by an optical detector for the return signals. The electronic return signals are demodulated and used for telephone and computer communications.
Those skilled in the art are directed to the following citations. U.S. Pat. No. 4,992,754 to Blauvelt discloses a pre-distortion network for compensating for second, third, and higher order distortion in a transmission device such as a semiconductor laser. U.S. Pat. No. 5,257,124 to Glaab discloses dual optical links to cancel out even order distortion. U.S. Pat. No. 5,430,568 to Little discloses a system in which 4 independent lasers each transmit different respective multi-carrier signals having different respective frequency bands of less than one octave each. At optical receivers, second order distortions are filtered out of each of the 4 signals and then the signals are combined into a single 54-500 MHz multi-carrier signal. Two pairs of lasers are used to transmit the 4 signals. For each pair of lasers, a first laser with a wavelength of 1310 nm transmits a first signal through a first fiber and a second laser with a wavelength of 1550 nm transmits a second signal through a second fiber; and wavelength division multiplexing (WDM) is used to combine the two signals from the first and second fiber into a first common 1310 nm zero dispersion fiber. Prior to reception, WDM is used to separate the first and second signals back into separate third and fourth fibers and separate respective receivers are provided to receive each signal. Electrical and/or optical compensating elements are provided to compensate for distortion due to dispersion which is not eliminated by grouping of frequencies discussed above. An electronic compensating element in the input of each laser need only compensate for third order distortion since the second order distortions are filtered out. Optical compensating elements in the second or third fiber compensate.for the dispersion of the 1550 nm signal in the 1310 nm zero dispersion fiber. The optical compensating elements may be dispersion compensating optical fibers having dispersion profiles opposite to the dispersion profile experienced by the optical signals when transmitted over standard 1310 nm optical fibers to the receiver location. Such profiles represent second and third order harmonic distortion, known in the art as composite second order and composite triple beat, respectively.
The above references are hereby incorporated herein in whole by reference.
In one embodiment of the invention herein, an electronic information signal is used to modulate a laser beam resulting in an optical signal that is transmitted through an optic fiber tree network to an optical detector that converts the optical signal back into an electronic information signal. A pre-compensation circuit distorts the electronic information signal prior to using the signal for modulating the laser beam. The pre-compensation circuit compensates for odd order distortion due to transmitting the optical signal through the optical fiber as well as the odd order distortion due to using the electronic signal for modulating the laser beam. The optical signal produced by the modulation of the laser beam is more distorted at the laser beam modulator than at an optical detector at a remote end of the optical fiber. As the distorted signal travels through the optical fiber it becomes less distorted. The pre-compensation circuit and the laser are components of an optical transmitter for transmitting an input information signal. The optical detector is a component of an optical receiver for outputting an electronic information signal that approximately duplicates the input information signal at the transmitter. The third order distortion of the pre-compensation circuit is selected so as to reduce the total odd order distortion in an electronic signal that is output from the optical receiver.
Preferably, the laser is directly modulated distributed feedback (DFB) laser which transmits at an optical wavelength selected between 1500 to 1610 nm and the optic fiber has approximately zero dispersion at approximately 1310 nm. The pre-compensation circuit also compensates for even order distortions due to the modulation of the laser beam and due to the transmission through the optic fiber.
Preferably, the pre-compensation circuit also compensates for distortions due to the optical detector converting the optical signals back into electronic signals. The pre-compensation circuit also compensates for electronic amplification required prior to laser modulation, electronic amplification required for the output electronic signal after optical detection, and optical amplification required for the optical signal.
In another embodiment of the.invention, a pre-compensation circuit compensates for odd order distortion due to transmitting an optical signal through an optical fiber, and the level of distortion provided by the pre-compensation circuit is adjustable depending on the length of the optical fiber.
Preferably, the pre-compensation circuit is automatically adjustable depending on a return signal from the receiver which is compared to a portion of the input electronic signal. Also, the pre-compensation circuit includes an input at a front panel for manually changing the level of distortion and an output from which the length of the fiber, for which the pre-compensation circuit is adjusted, can be determined.